A wireless device may use its current location for a variety of different purposes. For example, the wireless device's current location can be used to access one or more databases of shops, restaurants and other attractions within the vicinity of the wireless device, which can then be presented to a user of the wireless device. In another example, the wireless device's current location may be used to provide travel directions to the user of the wireless device. In another example, the wireless device's current location may be used in conjunction with emergency services, such when the wireless device's current location is provided to an operator as part of an E911 call. This list is not exhaustive, and the wireless device's current location may be used for many other purposes.
The Global Positioning System (“GPS”) is one coordinate-based location system that is commonly used by wireless devices to determine their respective locations. GPS is a satellite navigation system in which multiple satellites orbiting the earth transmit location information to wireless devices. The GPS satellites typically transmit low-power signals (e.g., approximately 20 W) to GPS receivers in the wireless devices. Depending on the time of day and the location of a wireless device, the wireless device may receive GPS signals from approximately four to eleven GPS satellites.
GPS satellites transmit using two different carrier signals—an L1 signal at 1575.42 MHz and an L2 signal at 1227.60 MHz. A coarse acquisition (“C/A”) code and a precise (“P”) code are modulated on the L1 carrier. The L2 code is modulated by either the P code or the C/A code, although typically the P code is used. The C/A code is a pseudo random binary code that repeats every 1023 bits, which is approximately every millisecond. Each GPS satellite uses a different C/A code, thereby allowing GPS receivers to distinguish between signals transmitted from different satellites. The P code is a very long pseudo random binary noise code, which repeats approximately every seven days.
GPS is available for both civilian and government use. The C/A codes are generally known, and they can be used in civilian applications to determine the GPS location of a wireless device. The P codes are generally not publicly known, and they may even optionally be encrypted to provide additional security. Therefore, the P codes are generally only used in military and other government applications. The P codes can be used in conjunction with the C/A codes to provide increased precision over a GPS location that is derived using only the C/A codes.
The L1 carrier is additionally modulated with a navigation data message. The navigation data message includes information describing the orbit of the GPS satellite, clock data and an approximate guide to the orbits of other GPS satellites. The GPS satellite transmits the navigation data message in 1500 bit data frames, which are further divided into five 300 bit subframes. A complete navigation data message takes twenty-five frames, which are sent by the GPS satellite over an approximately twelve and a half minute period.
The clock data in the navigation data message describes the GPS satellite's clock in relation to Universal Coordinated Time (“UTC”), an international time standard used by the GPS satellites and GPS receivers. The GPS satellite can embed within the data navigation message the time that each subframe was transmitted by the GPS satellite. The GPS receiver can then record the time that each subframe was received from the GPS satellite, thereby allowing the GPS receiver to determine the length of time it took the subframe to travel from the GPS satellite to the GPS receiver. Using this length of time and the position of the satellite, the GPS receiver can extrapolate its distance from the GPS satellite.
As a GPS receiver typically receives information from four or more GPS satellites, the GPS receiver can determine its distance from four known locations. Once the GPS receiver determines its distance from the four GPS satellites, it can extrapolate its GPS location in three dimensions. If the GPS receiver knows its distance from more than four GPS satellites, it can use this additional information to build redundancy and error correction into its location computation.
While GPS allows a wireless device to effectively determine its current location, GPS has limitations. Since GPS satellites transmit their location information using low-power signals, these signals are subject to atmospheric interference. The signals may additionally be degraded due to obstructions in the transmission path between the GPS satellite and the GPS receiver. For example, the 1575.42 MHz GPS signal does not penetrate well through buildings, trees, caves, cars or other such enclosures or obstructions.
As these signals do not penetrate well through enclosures, a wireless device that is not near a window or other opening in the enclosure may then have a difficult time detecting the GPS signals from the orbiting GPS satellites. As a wireless device moves further within the interior of the enclosure, this problem compounds. Consequently, a wireless device located within an enclosure might not be able to detect the GPS signals from the orbiting satellites and, therefore, would be unable to determine its GPS location at all.
Therefore, there exists a need for an improved system and method for allowing a device located within an enclosed area to determine its position in a coordinate-based location system.